U.S. patent number 4,532,477 [Application Number 06/565,185] was granted by the patent office on 1985-07-30 for distortion compensation for a microwave amplifier.
This patent grant is currently assigned to AT&T Bell Laboratories. Invention is credited to Donald R. Green, Jr., James P. Moffatt.
United States Patent |
4,532,477 |
Green, Jr. , et al. |
July 30, 1985 |
Distortion compensation for a microwave amplifier
Abstract
Amplifier-produced distortion known as amplitude modulation to
phase modulation (AM/PM) conversion is reduced through the use of
GaAs FETs which are biased to generate AM/PM having an algebraic
sign opposite to that generated by the amplifier. This algebraic
sign reversal is accomplished by biasing the GaAs FETs so that a DC
drain current .ltoreq.75% and .gtoreq.10% of the short-circuit
drain current is established. In the disclosed embodiment, several
GaAs FETs are cascaded in alternation with attenuators to increase
the magnitude of the compensating AM/PM conversion without
generating substantial amplitude modulation to amplitude modulation
(AM/PM) conversion.
Inventors: |
Green, Jr.; Donald R. (North
Andover, MA), Moffatt; James P. (Salem, NH) |
Assignee: |
AT&T Bell Laboratories
(Murray Hill, NJ)
|
Family
ID: |
24257543 |
Appl.
No.: |
06/565,185 |
Filed: |
December 23, 1983 |
Current U.S.
Class: |
330/149; 330/277;
330/290; 330/296 |
Current CPC
Class: |
H03F
1/306 (20130101); H03F 3/601 (20130101); H03F
1/3241 (20130101) |
Current International
Class: |
H03F
1/32 (20060101); H03F 1/30 (20060101); H03F
3/60 (20060101); H03F 001/32 (); H03F 003/16 () |
Field of
Search: |
;330/149,277,290,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mullins; James B.
Attorney, Agent or Firm: Padnes; David R.
Claims
What is claimed is:
1. Apparatus for use in a communication system wherein an amplifier
generates distortion known as amplitude modulation to phase
modulation conversion in an input signal, said apparatus
comprising
at least one GaAs field-effect transistor for receiving said
signal, said transistor having gate, source and drain terminals;
and
means for biasing said terminals independently of said input signal
so that said transistor generates amplitude modulation to phase
modulation conversion in said signal, said conversion having an
algebraic sign opposite to that generated in said signal by said
amplifier.
2. The apparatus of claim 1 wherein each of said transistors has a
short-circuit drain current and said biasing means establishes a DC
drain current .ltoreq.75% and .gtoreq.10% of the short-circuit
drain current.
3. The apparatus of claim 2 wherein said biasing means biases each
of said transistors for class A operation.
4. The apparatus of claim 3 further including attenuators.
5. The apparatus of claim 4 wherein said attenuators are cascaded
in alternation with said transistors.
6. The apparatus of claim 5 wherein each of said attenuators
reduces the drive level of an immediately succeeding one of said
transistors to minimize the generation of distortion known as
amplitude modulation to amplitude modulation conversion.
7. A method of reducing distortion known as amplitude modulation to
phase modulation conversion which is generated in a signal by an
amplifier, said method comprising the steps of
coupling said signal to at least one GaAs field-effect transistor
having gate, source, and drain terminals; and
biasing said terminals independently of said signal so that said
transistor generates amplitude modulation to phase modulation
conversion in said signal, said conversion having an algebraic sign
opposite to that generated in said signal by said amplifier.
8. The method of claim 7 wherein each of said transistors has a
short-circuit drain current and said biasing means establishes a DC
drain current .ltoreq.75% and .gtoreq.10% of the short-circuit
drain current.
9. The method of claim 8 wherein each of said transistors is biased
for class A operation.
10. The method of claim 9 further including the step of coupling
said signal through attenuators.
11. The method of claim 10 wherein said attenuators are cascaded in
alternation with said transistors.
12. The method of claim 11 wherein each of said attenuators reduces
the drive level of an immediately succeeding one of said
transistors to minimize the generation of distortion known as
amplitude modulation to amplitude modulation conversion.
13. Apparatus for use in a communication system wherein an
amplifier generates distortion known as amplitude modulation to
phase modulation conversion in an input signal, said apparatus
comprising
at least one single gate GaAs field-effect transistor for receiving
said signal, said transistor having gate, source and drain
terminals; and
means for biasing said terminals so that said transistor generates
amplitude modulation to phase modulation conversion in said signal,
said conversion having an algebraic sign opposite to that generated
in said signal by said amplifier.
14. A method of reducing distortion known as amplitude modulation
to phase modulation conversion which is generated in a signal by an
amplifier, said method comprising the steps of
coupling said signal to at least one single gate GaAs field-effect
transistor having gate, source and drain terminals; and
biasing said terminals so that said transistor generates amplitude
modulation to phase modulation conversion in said signal, said
conversion having an algebraic sign opposite to that generated in
said signal by said amplifier.
Description
TECHNICAL FIELD
The present invention relates to distortion compensation for
microwave amplifiers and, more particularly, to a technique which
compensates for the amplitude modulation to phase modulation
conversion generated by such amplifiers.
BACKGROUND OF THE INVENTION
Microwave amplifiers, such as solid-state power amplifiers and
traveling wave tube amplifiers, are widely used in communication
systems for transmitting analog or digital data. A chronic problem
with these amplifiers is that they exhibit nonlinear amplitude and
phase transfer characteristics. These distortions are primary
impediments to the reliable, spectrally-efficient transmission of
data.
For an amplifier input signal having a modulated amplitude, as in
many present-day communications systems, the nonlinear distortion
is categorized as amplitude modulation to amplitude modulation
(AM/AM) conversion and amplitude modulation to phase modulation
(AM/PM) conversion. AM/AM conversion relates to the amplitude
relationship at the amplifier input and output and can be defined
as the change in gain with respect to a change in the input or
output signal power. AM/PM conversion, on the other hand, relates
to the amplitude and phase characteristics of the amplifier and can
be defined as the change in the output signal phase relative to a
change in the input or output signal power.
AM/AM conversion is a function of the power handling capability of
the amplifier and can generally be lessened by reducing the drive
level of the amplifier so that the output power is consideraly
below saturation. This commonly used technique is known as
"amplifier back-off". Unfortunately, this technique often does not
adequately eliminate the AM/PM conversion. Consequently, in many
system applications, AM/PM conversion is the major portion of the
nonlinear distortion generated by microwave amplifiers.
Many prior art techniques which compensate for the nonlinear
distortion generated by microwave amplifiers (see, for example,
U.S. Pat. No. 3,755,754 to Putz and 4,283,684 to Satoh) have relied
on circuits comprising signal splitters, amplifiers, phase shifters
and signal combiners to generate a distortion correction signal.
This correction signal is then added to the microwave amplifier
input signal. A shortcoming of this technique is that it is often
times not amenable to integrated circuit techniques and, therefore,
the circuit design cannot be integrated onto a solid-state power
amplifier substrate. As a result, the distortion compensation
generated may not accurately track changes in the amplifier
transfer characteristic with changes in amplifier operating
temperature. In addition, the prior art circuits can be complex and
expensive to implement.
SUMMARY OF THE INVENTION
Distortion compensation circuitry in accordance with the present
invention comprises one or more gallium arsenide (GaAs)
field-effect transistors (FETs) which generate AM/PM conversion
having an opposite algebraic sign to the AM/PM conversion generated
by a microwave power amplifier. This algebraic sign reversal is
achieved by establishing an average or DC component of the drain
current which is less than 75% of the maximum or short-circuit
drain current in each FET. In the disclosed embodiment, the
distortion compensation circuitry comprises cascaded GaAs FETs
which together produce AM/PM conversion substantially equal and
opposite to the AM/PM conversion generated by a microwave power
amplifier. Advantageously, attenuators can be disposed between the
FETs to preclude saturation and the undesirable generation of AM/PM
conversion.
A feature of the present invention is that it can be incorporated
into a GaAs FET power amplifier and thereby provide distortion
compensation which tracks changes in the power amplifier operating
temperature.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block schematic diagram of the disclosed embodiment of
the present invention; and
FIG. 2 is a set of curves which depicts the relationship between
the drain current of a GaAs FET and AM/PM and AM/AM conversion.
DETAILED DESCRIPTION
As shown in FIG. 1, distortion compensation circuitry 10,
comprising cascaded, substantially identical GaAS FET stages 100,
116 and 117, is disposed in the input signal path of GaAs FET
microwave power amplifier 150. Such power amplifiers are typically
disposed in the transmitters of microwave communications systems.
The RF signal applied to input terminal 101 is a carrier signal
which is amplitude-modulated at a modulation frequency of at least
100 Kilohertz. For purposes of illustration, amplifier 150 is a
GaAs FET power amplifier which generates AM/PM conversion. That is,
the RF signal at the output of amplifier 150 is shifted in phase
relative to the RF signal at the input of amplifier 150 as a
function of input or output power. Furthermore, this AM/PM
conversion has an associated algebraic sign which depends on the
direction of the phase shift of the amplifier output signal
relative to the amplifier input signal. To reduce this AM/PM
conversion, distortion compensation circuitry 10 generates AM/PM
conversion in the input signal of amplifier 150 having an opposite
algebraic sign to the AM/PM conversion generated by amplifier 150.
This reversal of the algebraic sign is achieved by selection of the
DC gate-source bias voltage, V.sub.GS, which determines the DC
drain current in the GaAs FETs in stages 100, 116 and 117.
Concurrently, the DC drain-source bias voltage, V.sub.DS, is held
relatively constant in the saturated current region of the GaAs
FETs' common-source I-V characteristics. Moreover, while circuitry
10 comprises stages 100, 116 and 117, the number of stages can be
varied so that the magnitude of the AM/PM conversion provided by
circuitry 10 is substantially equal to the AM/PM conversion
generated by amplifier 150. Finally, when more than one GaAs FET
stage is utilized, RF attenuators 130 are advantageously disposed
between stages. Each RF attenuator reduces the drive level of the
immediately succeeding GaAs FET to substantially eliminate the
generation of AM/PM conversion by the distortion compensation
circuit 10.
To understand the relationship between the algebraic sign of the
AM/PM conversion generated by a GaAs FET and the DC component of
the drain current, refer now to FIG. 2. Curves 201, 202 and 203,
respectively, show the phase shift in degrees of a GaAs FET output
signal with respect to the input signal as a function of output
signal power for DC drain currents of 60, 75 and 100 milliamperes
(mA). The RF input signal used to generate these curves was a 6 GHz
carrier which was amplitude-modulated .+-.1 dB about some average
power level at a modulation frequency of 1 MHz. The maximum of
short-circuit drain current of the GaAs FET was 100 mA and,
therefore, curve 203 depicts the phase shift provided by a GaAs FET
power amplifier, such as amplifier 150. It should be noted that
regardless of the output power, the phase shift provided by the
GaAs FET is in one direction and is expressed in units of negative
degrees. In comparison, for a drain current of 60 mA, the phase
shift provided for an output power greater than 12.5 dBm is in a
direction opposite to that generated at 100 mA. This reversal in
the algebraic sign of the phase shift relative to 100 mA is also
true for a drain current of 75 mA when the output power level is
between 12.5 and 18 dBm.
Curves 204, 205 and 206 show the variations in gain as a function
of output signal power for 100, 75 and 60 mA, respectively. As
these curves depict that a substantially constant gain exists for a
range of output power levels, the GaAs FETs in amplifier 150 and
those in circuitry 10 can be easily operated within these ranges of
"flat" gain to minimize the generation of AM/AM conversion.
Refer back to FIG. 1. Distortion compensation circuitry 10
comprises several cascaded GaAs FETs each having an associated
biasing circuit. Each FET 115 is biased for class A operation with
the DC component of the drain current, I.sub.D, selected to be
.gtoreq.10% and .ltoreq.75% of the maximum or short-circuit drain
current. The upper percentage limit assures that the AM/PM
conversion generated by each FET is opposite in algebraic sign to
the AM/PM conversion generated by power amplifier 150. The lower
percentage limit assures that gain compression or AM/AM conversion
is virtually nonexistent.
The source terminal 114 of each FET 115 is grounded. The DC
voltages at drain terminal 113 and gate terminal 112 are provided
from a reference voltage source V.sub.DD using biasing circuitry
comprising adjustable resistor 104, operational amplifier 108 and
resistors 105, 106 and 107. Variable resistor 104 is adjusted to
set a preselected DC drain current, I.sub.D, which is .gtoreq.10%
or .ltoreq.75% of the maximum or short-circuit drain current of FET
115. This selection of I.sub.D also sets the voltage at the
positive input terminal 120 of amplifier 108. Resistors 105 and 106
form a voltage divider which establishes a fixed voltage at the
negative input terminal 121 of amplifier 108 which is greater than
the voltage at terminal 120. Resistor 107 sets the gain of
amplifier 108 so that gate terminal 112 of FET 115 is negatively
biased.
Advantageously, the disclosed biasing circuitry automatically
adjusts the DC bias voltage at the gate terminal to maintain the
preselected value of I.sub.D. For example, if I.sub.D decreases
after adjustment of resistor 104 due to temperature or other
effects, the bias voltage at gate terminal 112 becomes less
negative to restore the preselected value of I.sub.D. Similarly,
the bias voltage at gate terminal 112 becomes more negative so as
to decrease the value of I.sub.D if this current increases for any
reason after the adjustment of resistor 104.
RF chokes 103 and 109 prevent the RF input signal from entering the
bias circuitry. Capacitors 102 and 110, respectively, prevent the
DC component of the gate and drain bais currents from being coupled
to RF input terminal 101 and RF output terminal 111.
It should, of course, be understood that while the foregoing
description describes the distortion compensation of a GaAs FET
power amplifier, the present invention is equally applicable to
other RF amplifiers, such as klystrons and traveling wave tube
amplifiers. In addition, while circuitry 10 is disposed to
predistort the RF signal to a power amplifier, circuitry 10 can be
disposed at the output of an RF power amplifier to postdistort an
RF signal so as to reduce AM/PM conversion. Therefore, if
postdistortion is utilized, circuitry 10 can be located in either
the transmitter or receiver of a microwave communications
system.
* * * * *